In a semiconductor device communicating data wirelessly, high frequency electromagnetic waves transmitted from a reader/writer are received by an antenna to generate a power supply needed for circuit operation. In addition, data is transmitted by affecting electromagnetic waves in some way by a signal and modifying the amplitude, frequency, phase (also called position) and the like of the electromagnetic waves. Such a change in the amplitude and the like of the electromagnetic waves is called modulation. Electronic waves for transmitting signals are called carriers. A semiconductor device communicating data wirelessly is called a wireless IC card, an RFID tag, a wireless chip, a transponder, a wireless memory, an ID tag, an IC chip, or an ID card.
The rate of change of the amplitude of a modulated carrier is called a modulation depth. For example, a carrier with a modulation depth of 100% includes a state where the amplitude does not change (see FIG. 7A), while a carrier with a modulation depth of 10% includes a state where the amplitude is changed by 10% (see FIG. 7B). Note that a modulated carrier is hereinafter simply referred to as a carrier or a carrier including modulation. Further, a carrier with a modulation depth of 100% may be referred to as a carrier including 100% modulation.
According to a communication system defined by ISO/IEC 15693, which is the standard for vicinity wireless IC cards, data is coded using a pulse position modulation system where a carrier with a frequency of 13.56 MHz is modulated at 100% or 10% to change the pulse position and detect data. ISO/IEC 14443 (TYPE-A) and ISO/IEC 18000-3 are standards similar to the ISO/IEC 15693 standard. The ISO/IEC 14443 (TYPE-A) standard defines that 100% modulation has an amplitude 5% or less of an initial amplitude (amplitude in a state where a signal is not modulated).
One of the pulse position modulation systems defined by the aforementioned standards, which is called 4PPM (Pulse Position Modulation), is described with reference to FIG. 8A. FIG. 8A shows carrier waveforms of 2-bit data “00”, “01”, “10”, and “11”, and carrier waveforms of frame codes “SOF” and “EOF”. A line between rectangular black portions shows a pulse-modulated position of a carrier, and each carrier representing “00”, “01”, “10”, “11”, “SOF” and “EOF” has different pulse-modulated positions.
The duration of each carrier representing “00”, “01”, “10”, “11” and “SOF” is 75.2 μs, the duration of a carrier representing “EOF” is 37.76 μs, and the duration of the modulated portion of each carrier is 9.44 μs. The frame code “SOF” (Start Of Frame) is a signal representing the start of a frame, while the frame code “EOF” (End Of Frame) is a signal representing the end of a frame. The frame code “SOF” is a signal transmitted before each carrier representing “00”, “01”, “10” and “11” while the frame code “EOF” is a signal transmitted after each carrier representing “00”, “01”, “10” and “11”.
A flag signal and data such as a command are coded by the aforementioned pulse position modulation system, and the coded carrier is transmitted from a reader/writer to a semiconductor device. The semiconductor device demodulates the modulated carrier received from the reader/writer, and reads the pulse position to obtain data.
A common method for obtaining data of a semiconductor device is described with reference to FIG. 8B. Data obtained by the semiconductor device herein is a carrier modulated at 100%, which represents “00”, “01”, “10” and “11” and uses “SOF” as a start signal.
The semiconductor device uses a clock signal to obtain data. The clock signal herein is a signal synchronized with a carrier modulated at 100%, and a half period of the clock signal is equal to the modulation width. The semiconductor device has a counter for counting two bits of count 1 and count 2 using a clock signal. The counter repeatedly counts from “00” to “11”, provided that “00” is the timing at which “SOF” is modulated at 100% for the first time. The timing of modulating each carrier at 100% corresponds to the counter value. Accordingly, the semiconductor device demodulates the modulated carrier and reads the pulse position to obtain data by using the counter value as the timing of modulating the carrier at 100%.
A carrier representing the data “00” is modulated at 100% when the counter counts “00”, and a carrier representing the data “01” is modulated at 100% when the counter counts “01” (see FIG. 8B). Similarly, a carrier representing the data “10” is modulated at 100% when the counter counts “10”, and a carrier representing the data “11” is modulated at 100% when the counter counts “11”. Thus, the semiconductor device can obtain the data “00”, “01”, “10” and “11” by using the counter value as the timing of modulating each carrier at 100%.
As described above, the semiconductor device demodulates a modulated carrier and reads the pulse position to obtain data by using the clock signal and the counter. However, a signal received by the semiconductor device from an antenna is only a carrier and a demodulated signal obtained by demodulating the carrier. Therefore, it is necessary to generate a clock signal for detecting the pulse position in the semiconductor device.
In order to generate a clock signal in a semiconductor device, a PLL (Phase Locked Loop) circuit may be provided in the semiconductor device. The PLL circuit detects a phase difference between an input signal and an output signal and controls a voltage controlled oscillator (VCO) for generating an output signal, so that an output signal with a frequency accurately synchronized with an input signal can be obtained. By providing the PLL circuit in the semiconductor device, a waveform synchronized with a carrier or a demodulated signal can be obtained to generate a clock signal utilized for internal operation.